What is DTD?
DTD, short name for Document Type Definition, is used as the file extension for Document Type Definition files.
.DTD files are text files that can be opened by Notepad or any text editors.
See the sample .DTD file included below.

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A

File Extension: .DTD

MIME Type: text/plain

File Content: Document Type Definition

.DTD files are Document Type Definition files, which are used to provide
element and attributes definitions for a group of XML files.
Here is a sample .DTD Document Type Definition file, image.dtd: